In liquid crystal displays, various optical films are used for improving the display performance. For example, an optical film (retardation film) that satisfies the condition (1) below is arranged between a liquid crystal cell and a polarizing plate, and used for compensating the viewing angle of a liquid crystal display. Such an optical film is produced by stretching a polymer film (see JP H03(1991)-33719 A, JP H03(1991)-24502 A, and JP H04(1992)-194820 A).nx≧ny>nz  (1)
In the above formula (1), nx, ny and nz indicate respectively refractive indices in directions of an X-axis, a Y-axis, and a Z-axis in the optical film. The X-axis direction is a direction showing a maximum refractive index within the plane of the optical film, the Y-axis direction is a direction perpendicular to the X-axis direction within the plane, and the Z-axis direction is a thickness direction and is perpendicular to the X- and Y-axes directions.
However, in a producing method that includes a stretching process, conditions regarding the stretching ratio and stretching direction or the like must be set in detail, which will require a precise control of the stretching and thus the processes will be complicated. Moreover in the stretching, a bowing phenomenon must be solved. Furthermore, the stretching requires use of a polymer film of a certain degree of thickness. The thus obtained optical film will be thick, and as a result, the liquid crystal display will be thick as well.
In some other methods, optical films can be produced without a stretching treatment. Such an optical film can be produced, for example, by applying a polymer solution onto a flat-surface base and removing the solvent of the solution by evaporation. Examples of the polymer include polyamide, polyimide, polyester, polyetherketone, polyamideimide, polyesterimide and the like (see U.S. Pat. No. 5,344,916, U.S. Pat. No. 5,395,918, JP H08(1996)-511812 A, U.S. Pat. No. 5,480,964, U.S. Pat. No. 5,580,950, and U.S. Pat. No. 6,074,709) For the base, an inorganic base such as a SUS belt, a copper thin sheet, a glass sheet, a Si wafer or the like are used mainly. However, since the inorganic base itself cannot be used in a liquid crystal display, the optical film formed on the base must be transferred onto a polarizer or the like. Alternatively, the optical film formed on the base will be peeled off and rolled. As a result, use of an inorganic base will complicate a process for producing an optical film, and the inorganic base may increase the cost.
As mentioned above, there are problems in the method for producing conventional optical films. Moreover, the optical films are required to have further improved optical properties.